Developing green propulsion concepts for aviation requires some changes in standard procedures for the preliminary aircraft design. Standard design procedures are not readily applicable to aircraft/rotorcraft that use multiple energy sources; innovative concepts like hybrid propulsion systems require new approaches in weight and performance prediction. In this paper the application of new methods is explored to account for the changes in the governing equations when using hybrid propulsion: a design code allows predicting the performances once the hybrid propulsion architecture is chosen. Focus is on hybrid propulsion systems based on a diesel heavy fuel ICE (Internal Combustion Engine) combined with an electric motor in a parallel configuration. This study relies on simple structure and material performance indexes, neglects stability and control analysis, and uses very simple aerodynamic models.